back up desktop, documents, files to iCloud

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

iCloud Drive FAQ - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support

Is your system version shown below your question still valid?

OS X 10.10 does not yet support the syncing of Desktop and Documents. You would have to upgrade to a newer system version, at least macOS 10.12 Sierra.

If you want to sync with iCloud for an off-site storage and still be able to have a backup of your documents and data, do not enable the option "Optimise Storage" to save local storage on your Mac. If you do tat, none of the easy backup solutions like Time Machine or cloning software will be able to keep backups of all documents and data, because you will no longer have local copies of all documents and data, so the local backup mentors can no longer access them. You will have to download and save the iCloud documents and data manually to be able to have a backup archive of older versions. See: Archive or make copies of the information you store in iCloud - Apple Support

iCloud is a syncing service, not an 'external hard drive'. I suggest you sync to iCloud and backup with Time Machine.

To do what you want to do, go to System Preferences/iCloud/iCloud Drive/Options and check Desktop & Documents Folders. Then go to System Preferences/Time Machine, click Options, and add your Desktop and Documents folder.